The Pentagon this week approved a major policy directive that elevates the military's mission of "irregular warfare" -- the increasingly prevalent campaigns to battle insurgents and terrorists, often with foreign partners and sometimes clandestinely -- to an equal footing with traditional combat.

The directive, signed by Deputy Defense Secretary Gordon England on Monday, requires the Pentagon to step up its capabilities across the board to fight unconventionally, such as by working with foreign security forces, surrogates and indigenous resistance movements to shore up fragile states, extend the reach of U.S. forces into denied areas or battle hostile regimes.

The policy, a result of more than a year of debate in the defense establishment, is part of a broader overhaul of the U.S. military's role as the threat of large-scale combat against other nations' armies has waned and new dangers have arisen from shadowy non-state actors, such as terrorists that target civilian populations.

"The U.S. has considerable overmatch in traditional capabilities . . . and more and more adversaries have realized it's better to take us on in an asymmetric fashion," said Michael G. Vickers, assistant secretary of defense for special operations/low-intensity conflict and interdependent capabilities, and a chief architect of the policy.

Designed to institutionalize lessons the U.S. military has learned -- often painfully -- in Afghanistan and Iraq since 2001, the policy aims to prepare the military for the most likely future conflicts and to prevent the type of mistakes made in the post-Vietnam War era, when hard-won skills in counterinsurgency atrophied.

Defense Secretary Robert M. Gates has lobbied outspokenly for such a shift.

"Think of where our forces have been sent and have been engaged over the last 40-plus years: Vietnam, Lebanon, Grenada, Panama, Somalia, Haiti, Bosnia, Kosovo, Afghanistan, Iraq, the Horn of Africa and more," Gates said in a recent speech at the National Defense University. "In fact, the first Gulf War stands alone in over two generations of constant military engagement as a more or less traditional conventional conflict."

Gates warned that, for the near future, the United States will face the greatest threats not from aggressor countries but from insurgents and extremist groups operating in weak or failing states. "We do not have the luxury of opting out because they do not conform to preferred notions of the American way of war," he said.

The new, 12-page directive states that irregular warfare "is as strategically important as traditional warfare."

Defined as "a violent struggle among state and non-state actors for legitimacy and influence over the relevant population(s)," irregular warfare "favors indirect and asymmetric approaches . . . to erode an adversary's power, influence, and will," the directive states.

A major thrust of the policy is for U.S. troops to do less of the fighting themselves and instead build the capabilities of foreign militaries and security forces.

Indeed, Vickers said he envisions that the Pentagon's primary vehicle for carrying out irregular warfare operations will be a global network -- already underway -- made up of the U.S. and foreign militaries and other government personnel in scores of countries with which the United States is not at war. The network is designed to wage "steady state" counterterrorism operations. The directive also requires the Pentagon to develop capabilities to conduct larger-scale irregular campaigns, such as those in Iraq and Afghanistan.

The goal of the network, Vickers said in a recent speech, is ambitious: "to create a persistent, ubiquitous presence against our adversaries . . . and essentially to smother them over time."

The directive "should have a big impact on resources" as well as military planning, Vickers said.

Specifically, as irregular warfare is more manpower-intensive, it is likely to shift more resources toward training the Army and Marine Corps, which are undergoing significant growth, in skills such as language learning and advising foreign militaries, he said.

The policy also supports continued growth in Special Operations forces -- elite troops such as Army Green Berets skilled in partnering with foreign forces and civil affairs soldiers who conduct nation-building.

As irregular warfare is likely to be conducted by Special Operations forces, the policy directs the U.S. Special Operations Command, based in Tampa, to "develop capabilities for extending U.S. reach into denied areas and uncertain environments by operating with and through indigenous foreign forces or by conducting low visibility operations."

In terms of equipment, the directive supports the expansion of intelligence, reconnaissance and surveillance assets, as well as aviation assets for irregular warfare, Vickers said.